Carbon nanotubes as catalysts in redox reactions

ABSTRACT

Carbon nanotubes have been reversibly and readily oxidized and reduced with common chemicals in solution, thereby allowing the nanotubes to be used as catalysts for chemical reactions and as stable charge storage devices.

FIELD OF INVENTION

This invention describes a new type of reversible redox reaction ofcarbon nanotubes in aqueous solution.

BACKGROUND

Carbon nanotubes (CNT) have been the subject of intense research sincetheir discovery in 1991. CNT's possess unique properties such as smallsize, considerable stiffness, and electrical conductivity, which makesthem suitable in a wide range of applications, including use asstructural materials and in molecular electronics, nanoelectroniccomponents, and field emission displays. Carbon nanotubes may be eithermulti-walled (MWNTs) or single-walled (SWNTs), and have diameters in thenanometer range.

Little work has been done on the unique chemical properties of carbonnanotubes in solution phase. As an extended conjugated double-bondsystem with high surface area, carbon nanotubes are expected tostabilize and accumulate charges far better than small molecules. Theaccumulated charges on a carbon nanotube can be potentially used forredox reactions that are hard to carry out using small molecules.

Methods of manipulating charges on carbon nanotubes by chemical dopingand electrochemical control have been reported (Petit, P., et al,Chemical Physics Letters 305, 370-374 (1999); Kavan, L., et al.,Chemical Physics Letters 328, 363-368 (2000)). While chemical dopingmethods and electrochemical processes are conceptually identical tooxidation and reduction, there is a fundamental difference between whathas been reported and our method. All the previous experiments wereconducted in solid phase with carbon nanotubes mounted on a substrate.

Strano and collaborators have reported a marked sensitivity of theoptical transitions to pH in carbon nanotubes dispersed by a surfactant(Strano, M. S. et al. Journal of Physical Chemistry B 107, 6979-6985(2003)). They further showed that the pH effect was dependent on thepresence of O₂.

Applicants have shown that dispersed carbon nanotubes can be reversiblyand readily oxidized and reduced with common chemicals in solution,thereby allowing the carbon nanotubes to be used as catalysts forchemical reactions and for photoelectrochemical reactions if chargeseparation is created by light absorption.

SUMMARY OF THE INVENTION

The invention provides a stable charge storage device comprising atleast one oxidized carbon nanotube in solution. Particularly provided isa stable charge storage device where the carbon nanotube is in the formof dispersed nanotubes

In another embodiment the invention provides a process to catalyze aredox reaction comprising: contacting at least one carbon nanotube with:a) an oxidizing agent and an oxidizable compound or b) a reducing agentand a reducible compound, wherein the redox reaction is catalyzed.

Also provided is a process for altering the reduction potential of acarbon nanotube in solution comprising contacting the carbon nanotubewith a dispersant.

Also provided is a process for selecting the reduction potential of acarbon nanotube comprising: a) determining the diameter of a carbonnanotube; and b) correlating the diameter of step (a) with the reductionpotential of the carbon nanotube.

Also provided is a process for altering the number of valence electronsin a carbon nanotube comprising contacting a carbon nanotube with anoxidizing agent or a reducing agent.

Also provided is a stable charge transfer reagent comprising at leastone oxidized or reduced carbon nanotube in solution.

Also provided is a pH sensor comprising at least one oxidized or reducedcarbon nanotube in solution.

Also provided is a photovoltaic cell comprising a film comprising carbonnanotubes.

Also provided is a photovoltaic cell comprising: a) an opticallytransparent, electronically conductive substrate; b) a film comprisingcarbon nanotubes in contact with a); c) an electrolyte comprising aredox reagent in contact with b); and d) an electrically conductivecathode in contact with c.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 a shows the redox titration of (6,5) enriched carbon nanotubes byK₂IrCl₆ as followed by the UV-Vis spectroscopy.

FIG. 1B shows the redox titration of HIPCO carbon nanotubes by differentconcentrations of K2IrCl6 as followed by the UV-Vis spectroscopy.

FIG. 2 shows the consumption of KMnO₄ in the presence and in the absenceof CNT monitored at 526 nm.

FIG. 3 shows the spectral changes of a KMnO₄ and CNT reaction mixture asa function of time as followed by the UV-Vis spectroscopy.

FIGS. 4 a and 4 b show the titration by K₂IrCl₆ of carbon nanotubesdispersed with RNA and with Triton X-405 as followed by UV-Visspectroscopy.

FIG. 5 shows the effect of reductant on RNA-wrapped CNTs as followed bythe UV-Vis spectroscopy.

FIG. 6A shows the pH titration of CNTs at 1290 nm with 1% Triton X-405.

FIG. 6B shows the pH titration of CNTs at 1290 nm with 1% TritonX-405+0.1% SDS.

FIG. 7 is a diagram of a possible embodiment of a carbon nanotube basedphotovoltaic cell.

DETAILED DESCRIPTION OF THE INVENTION

The invention is related to a stable charge storage device and to acharge transfer reagent comprising at least one oxidized carbon nanotubein solution. It has been found that carbon nanotubes can be reversiblyand readily oxidized and reduced with common chemicals in solution,enabling the storage of charges that are stable, and that can betransferred.

In this disclosure the following terms and abbreviations may be used forthe interpretation of the claims and specification.

“cDNA” means complementary DNA

“PNA” means peptide nucleic acid

“ssDNA” means single stranded DNA

“tRNA” means transfer RNA

“CNT” means carbon nanotube

“MWNT” means multi-walled nanotube

“SWNT” means single walled nanotube

For the purposes of this invention, “oxidized” refers to the state of asubstance that has lost one or more negative charges in the form ofelectrons. “Reduced” refers to the state of a substance that has gainedone or more negative charges in the form of electrons. A “redox”reaction refers to a reaction in which at least one substance isoxidized and at least one substance is reduced. “Oxidizing agent” isdefined as a substance that readily accepts electrons in a redoxreaction. “Reducing agent” is defined as a substance that readilydonates electrons in a redox reaction. An “oxidizable compound” is asubstance that can be oxidized by an oxidizing agent. A “reduciblecompound” is a substance that can be reduced by a reducing agent.“Reduction potential” is the relative tendency of a compound to act asan electron donor or acceptor and is usually measured as the redoxpotential at which a species is oxidized or reduced under standardconditions. Typically the reduction potential is relative to thestandard hydrogen electrode.

As used herein the term “charge storage device” refers to a device ormaterial that has the ability to accumulate and store either a positiveof negative charge. Preferred storage devices of the invention arecarbon nanotubes in solution.

As used herein the term “charge transfer reagent” refers to a materialthat that has the ability to transfer either a positive or negativecharge from one material to another. Preferred charge transfer reagentsof the invention are carbon nanotubes in solution.

By “in solution” it is meant for the purposes of this invention that thenanotubes are either dissolved or suitably dispersed in the solvent. Anymethod known in the art can be used to dissolve or disperse thenanotubes. Any solvent or mixture of solvents can be used provided thatit is inert to all reagents and products. Examples of solvents that canbe used include water and organic solvents such as but not limited todichlorobenzene, methanol, ethanol, and isopropanol.

As used herein a “nucleic acid molecule” is defined as a polymer of RNA,DNA, or peptide nucleic acid (PNA) that is single- or double-stranded,optionally containing synthetic, non-natural or altered nucleotidebases. A nucleic acid molecule in the form of a polymer of DNA may becomprised of one or more segments of cDNA, genomic DNA or synthetic DNA.

The letters “A”, “G”, “T”, “C” when referred to in the context ofnucleic acids will mean the purine bases adenine (C₅H₅N₅) and guanine(C₅H₅N₅O) and the pyrimidine bases thymine (C₅H₆N₂O₂) and cytosine(C₄H₅N₃O), respectively.

The term “peptide nucleic acids” refers to a material having stretchesof nucleic acid polymers linked together by peptide linkers.

As used herein the term “stabilized solution of nucleic acid molecules”refers to a solution of nucleic acid molecules that are solubilized andin a relaxed secondary conformation.

The invention provides a charge storage device which consists of leastone oxidized or reduced carbon nanotube in solution

Carbon Nanotubes

The term “carbon nanotube” refers to a hollow article composed primarilyof carbon atoms. The carbon nanotube can be doped with other elements,e.g., metals. The nanotubes typically have a narrow dimension (diameter)of about 1-200 nm and a long dimension (length), where the ratio of thelong dimension to the narrow dimension, i.e., the aspect ratio, is atleast 5. In general, the aspect ratio is between 10 and 2000.

Carbon nanotubes of the invention are generally about 0.5-2 nm indiameter where the ratio of the length dimension to the narrowdimension, i.e., the aspect ratio, is at least 5. In general, the aspectratio is between 10 and 2000. Carbon nanotubes are comprised primarilyof carbon atoms, however they may be doped with other elements, e.g.,metals. The carbon-based nanotubes of the invention can be eithermulti-walled nanotubes (MWNTs) or single-walled nanotubes (SWNTs). AMWNT, for example, includes several concentric nanotubes each having adifferent diameter. Thus, the smallest diameter tube is encapsulated bya larger diameter tube, which in turn, is encapsulated by another largerdiameter nanotube. A SWNT, on the other hand, includes only onenanotube.

Carbon nanotubes (CNT) may be produced by a variety of methods, and areadditionally commercially available. Methods of CNT synthesis includelaser vaporization of graphite (A. Thess et al. Science 273, 483(1996)), arc discharge (C. Journet et al., Nature 388, 756 (1997)) andHiPCo (high pressure carbon monoxide) process (P. Nikolaev et al. Chem.Phys. Lett. 313, 91-97 (1999)). Chemical vapor deposition (CVD) can alsobe used in producing carbon nanotubes (J. Kong et al. Chem. Phys. Lett.292, 567-574 (1998); J. Kong et al. Nature 395, 878-879 (1998); A.Cassell et al. J. Phys. Chem. 103, 6484-6492 (1999); H. Dai et al. J.Phys. Chem. 103, 11246-11255 (1999)).

Additionally CNT's may be grown via catalytic processes both in solutionand on solid substrates (Yan Li, et al., Chem. Mater.; 2001; 13(3);1008-1014); (N. Franklin and H. Dai Adv. Mater. 12, 890 (2000); A.Cassell et al. J. Am. Chem. Soc. 121, 7975-7976 (1999)).

Most CNT's, as presently prepared, are in the form of entangled tubes.Individual tubes in the product differ in diameter, chirality, andnumber of walls. Moreover, long tubes show a strong tendency toaggregate into “ropes” held together by Van der Waals forces. Theseropes are formed due to the large surface areas of nanotubes and cancontain tens to hundreds of nanotubes in one rope.

Dispersion of Carbon Nanotubes

The carbon nanotubes may be dispersed in the solution by any means knownin the art, such as but not limited to dispersion with detergents andsurfactants such as sodium dodecylsulfate, alkyl benzene sulfonate,dextrin, polyethylene oxide, alkyl-ether sulfonate and Triton® seriescompounds, ultrasonication, wrapping with polymers such aspolyvinylpyrrolidone, polysaccharides, polypeptides, gum arabic, andpolystyrene sulfonate, and chemical functionalization of the nanotubesurface. The dispersants used may be charged or uncharged. Many of thesetechniques are reviewed in Hilding, J., et al., Journal of DispersionScience and Technology (2003), 24(1), 1-41. More than one dispersant canbe used in the same process, including dispersants of different charges,such as but not limited to an uncharged dispersant mixed with an anionicdispersant.

Another method to disperse CNT aggregates is by the use of stabilizedsolutions of nucleic acid molecules, as described in U.S. patentapplication Ser. Nos. 10/716,346 and 10/716,347, incorporated herein byreference. They describe a method for dispersing a population of bundledcarbon nanotubes by contacting the bundled nanotubes with a stabilizedsolution of nucleic acid molecules which comprises:

a) providing a stabilized solution of nucleic acid molecules;

b) contacting a population of carbon nanotubes with an effective amountof the stabilized nucleic acid solution of step (a) for a timesufficient to disperse the carbon nanotubes; and

c) optionally recovering the dispersed carbon nanotubes.

The nucleic acid molecules may be of any type and from any suitablesource and include but are not limited to DNA, RNA and peptide nucleicacids. The nucleic acid molecules may be either single stranded ordouble stranded and may optionally be functionalized at any point with avariety of reactive groups, ligands or agents. The nucleic acidmolecules of the invention may be generated by synthetic means or may beisolated from nature by protocols well known in the art (Sambrook, J.,Fritsch, E. F. and Maniatis, T., Molecular Cloning: A Laboratory Manual,Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor,N.Y. (1989)).

Peptide nucleic acids (PNA) may also be used as they possess the doublefunctionality of both nucleic acids and peptides. Methods for thesynthesis and use of PNA's are well known in the art, see for exampleAntsypovitch, S. I. Peptide nucleic acids: structure Russian ChemicalReviews (2002), 71(1), 71-83.

The nucleic acid molecules may have any composition of bases and mayeven consist of stretches of the same base (poly A or poly T forexample) without impairing the ability of the nucleic acid molecule todisperse the bundled nanotube. Preferably the nucleic acid moleculeswill be less than about 2000 bases where less than 1000 bases ispreferred and where from about 5 bases to about 1000 bases is mostpreferred. Generally the ability of nucleic acids to disperse carbonnanotubes appears to be independent of sequence or base composition,however there is some evidence to suggest that the less G-C and T-Abase-pairing interactions in a sequence, the higher the dispersionefficiency, and that RNA and varieties thereof is particularly effectivein dispersion and is thus preferred herein. Nucleic acid moleculessuitable for use in the present invention include but are not limited tothose having the general formula:

-   -   1. An wherein n=1-2000;    -   2. Tn wherein n=1-2000;    -   3. Cn wherein n=1-2000;    -   4. Gn wherein n=1-2000;    -   5. Rn wherein n=1-2000, and wherein R may be either A or G;    -   6. Yn wherein n=1-2000, and wherein Y may be either C or T;    -   7. Mn wherein n=1-2000, and wherein M may be either A or C;    -   8. Kn wherein n=1-2000, and wherein K may be either G or T;    -   9. Sn wherein n=1-2000, and wherein S may be either C or G;    -   10. Wn wherein n=1-2000, and wherein W may be either A or T;    -   11. Hn wherein n=1-2000, and wherein H may be either A or C or        T;    -   12. Bn wherein n=1-2000, and wherein B may be either C or G or        T;    -   13. Vn wherein n=1-2000, and wherein V may be either A or C or        G;    -   14. Dn wherein n=1-2000, and wherein D may be either A or G or        T; and    -   15. Nn wherein n=1-2000, and wherein N may be either A or C or T        or G;

In addition to the combinations listed above the person of skill in theart will recognize that any of these sequences may have one or moredeoxyribonucleotides replaced by ribonucleotides (i.e., RNA or RNA/DNAhybrid) or one or more sugar-phosphate linkages replaced by peptidebonds (i.e. PNA or PNA/RNA/DNA hybrid).

Once the nucleic acid molecule has been prepared it must be stabilizedin a suitable solution. It is preferred if the nucleic acid moleculesare in a relaxed secondary conformation and only loosely associated witheach other to allow for the greatest contact by individual strands withthe carbon nanotubes. Stabilized solutions of nucleic acids are commonand well known in the art (see Sambrook supra) and typically includesalts and buffers such as sodium and potassium salts, and TRIS(Tris(2-aminoethyl)amine), HEPES(N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid), andMES(2-(N-Morpholino)ethanesulfonic acid. Preferred solvents forstabilized nucleic acid solutions are those that are water misciblewhere water is most preferred.

Once the nucleic acid molecules are stabilized in a suitable solutionthey may be contacted with a population of bundled carbon nanotubes. Itis preferred, although not necessary if the contacting is done in thepresence of an agitation means of some sort. Typically the agitationmeans employs sonication for example, however may also include, devicesthat produce high shear mixing of the nucleic acids and nanotubes (i.e.homogenization), or any combination thereof. Upon agitation the carbonnanotubes will become dispersed and will form nanotube-nucleic acidcomplexes comprising at least one nucleic acid molecule looselyassociated with the carbon nanotube through van der Waals interactionsand possibly aided by hydrogen bonding.

The process of agitation and dispersion may be improved with theoptional addition of nucleic acid denaturing substances to the solution.Common denaturants include but are not limited to formamide, urea andguanidine. A non-limiting list of suitable denaturants may be found inSambrook supra.

Catalysis of Redox Reactions

The invention is also directed to a process to catalyze a redox reactioncomprising: contacting at least one carbon nanotube with: a) anoxidizing agent and an oxidizable compound or b) a reducing agent and areducible compound, wherein the redox reaction is catalyzed.

The invention is also directed to a process for altering the number ofvalence electrons in a carbon nanotube comprising contacting a carbonnanotube with an oxidizing agent or a reducing agent.

One of the advantages of the nanotube as a catalyst is its large surfacearea relative to that of the redox reactants. By having each redoxreactant interact with the nanotube as well as with each other, the rateof electron exchange between the reactants is increased. Oxidizingequivalents generated by a molecule at one location along a nanotube maybe used to oxidize another molecule at a distance, as diagrammed below:

Examples of oxidizing agents suitable for used in the instance inventioninclude, but are not limited to,2,3-dichloro-5,6-dicyano-p-benzoquinone, benzoyl peroxide, chloranil,chlorine, dichlorodicyanobenzoquinone, dichlorodicyano-p-benzoquinone,diphenylbenzoquinone, divalent chromium, formate, glucose,hexachloroiridate, hydrogen peroxide, hydroperoxide, hyperchlorous,hypophosphite, iodine, naphthoquinone, N-bromosuccinimide,N-chlorosuccinimide, N-iodosuccinimide, nitrate, nitric acid, nitrousacid, o-benzoquinone, o-chloranil, o-iodobenzoate, oxygen, ozone,p-benzoquinone, p-chloranil, peracetic acid, perborate, perboric acid,perchlorate, perchloric acid, permanganate, permanganic acid, peroxide,peroxyacids, perphosphoric acid, persulfate, persulfuric acid,pertitanate, pervanadate, p-toluquinone, ribose, saccharose, perforate,stannous, stilbenequinone, sulfite, sulfur, sulfuric acid,tetrathionate, trivalent titanium, and trivalent vanadium.

Examples of reducing agents include, but are not limited to,9,10-diphenylanthracene radical anion, acid chlorides, acridine anion,alkali metal sulfates, aluminum sulfates, anthraquinol, benzophenoneanion, bisulfite, boranes, borates, carbon monoxide, chromium(II) salts,diimine, dithionite, formic acid, Group I & II metals, hydrazine,hydrides, hydrogen, hydrosulfite, iron(II) salts, lithium aluminumhydride, low-valence titanium ions, naphthalene radical-anion, nitrousacid, N-n-butylphthalimide anion, niobium(III) salts, perylene radicalanion, phenanthridine radical-anion, phosphates, phosphines,phthalonitrile anion, polyphosphates, reducing sugars, sodiumborohydride, sulfides, thiosulfate, tranisition metals, trans-stilbeneanion, and vanadium(II) salts.

In order to serve as a catalyst the carbon nanotube of the inventionwill generally be in solution in the presence of redox reactants.Typically the solution will be aqueous based and at an acid pH where anypH of less than 7.0 is suitable. Alternatively organic solvents may beemployed either alone or admixed with water. Suitable organic solventswill include, but are not limited to dichlorobenzene, methanol, ethanol,and isopropanol.

Tuning of Reduction Potential

One of the distinct advantages for the use of a carbon nanotube as acharge storage device is that fact that it will be possible to “tune” orcalibrate the reduction potential of the catalyst to accommodate thespecific needs of any particular redox reaction. It has been noted forexample that the present CNT's may be dispersed in a variety of waysincluding in detergents and by wrapping the CNT in polymers,particularly nucleic acids. It has been observed that the reductionpotential of the catalyst may be altered depending on the type ofdispersant used. For example CNT's dispersed in neutral detergents areexpected to have a reduction potential of about 1100 mv to about 500 mv,whereas the reduction potential of a CNT wrapped in a nucleic acid isexpected to be about 900 mv to about 500 mv.

Alternatively the calibration of the reduction potential may beaccomplished by selecting CNT's of a particular diameter. Physicalcharacterization such as the determination of CNT diameter is easilymeasured by means such as Resonance Raman spectroscopy, scanningtunneling microscopy, the methods of which are reviewed in PhysicalProperties of Carbon Nanotubes by R. Saito, G Dresselhaus and M. S.Dresselhaus (Imperial College Press). For example it has been observedthat CNT's having a relatively small diameter (from about 0.7 nm toabout 0.8 nm for example) have a relatively high reduction potential(from about 820 nm to about 780 mv), whereas those CNT's having arelative large diameter (from about 1.0 nm to about 1.1 nm) will have alower reduction potential (from about 760 mv to about 720 mv).

Thus the invention is directed to a process for altering the reductionpotential of a carbon nanotube in solution comprising contacting thecarbon nanotube with a dispersant, and to a process for selecting thereduction potential of a carbon nanotube comprising: a) determining thediameter of a carbon nanotube; and b) correlating the diameter of step(a) with the reduction potential of the carbon nanotube.

The person of skill in the art will recognize that this ability tocalibrate reduction potential either by the use of dispersants or byselecting for morphological characteristics of the nanotube may beexpanded upon given the teaching of the invention and are notnecessarily limited to the specific instances mentioned here.

Photovoltaic cells using nanoparticles and conducting films have beendescribed in the art (A. Hagfeldt, Acc. Chem. Res. 2000, 33, 269-277 andM Grätzel, Nature, Vol 414, 15 Nov. 2001, 338-342). The carbon nanotubesof the present invention that are able to be reversibly oxidized andreduced can be used in such a device.

One embodiment of a photovoltaic cell comprising carbon nanotubes is alayered assemblage as shown in FIG. 7. An optically transparent,electronically conductive substrate (10), which is transparent toelectromagnetic radiation, such as but not limited to visible,ultraviolet, near-ultraviolet, infrared, and/or near-infraredwavelengths; is coated with or in contact with a film comprising thecarbon nanotubes (20). The film can be prepared from carbon nanotubes orprepared from another substrate containing carbon nanotubes. Anelectrolyte (40) comprising a redox mediator (50) is in electricalcontact with the carbon nanotube (20). The electrolyte may be eithersolid or liquid. An electrically conductive cathode (30) in electricalcontact with the electrolyte (40). In use, the electromagnetic radiation(R) passes through conducting substrate (10) and is absorbed by film(20). An electrical potential is produced across the cell, which can beused to power a load (L).

EXAMPLES

The present invention is further defined in the following Examples. Itshould be understood that these Examples, while indicating preferredembodiments of the invention, are given by way of illustration only.From the above discussion and these Examples, one skilled in the art canascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various uses andconditions.

The meaning of abbreviations is as follows: “h” means hour(s), “min”means minute(s), “sec” means second(s), “d” means day(s), “mL” meansmilliliters, “L” means liters, “W” means watts.

Example 1 Obtaining (6,5) Enriched Carbon Nanotubes by Anion ExchangeChromatography

This example describes preparation of carbon nanotube materials to beused for experiments in the subsequent EXAMPLES. The procedure followedis described in U.S. patent application Ser. No. 10/716,347. Unpurifiedsingle wall carbon nanotubes from Southwest Nanotechnologies (SWeNT,Norman, Okla.) and single-stranded DNA (GT)30 (Integrated DNATechnologies, INC, Coralville, Iowa) were used. Dispersion andseparation by anion exchange chromatography were done as described inU.S. patent application Ser. No. 10/716,347. This procedure yielded afraction that was largely enriched with (6,5) tubes.

Example 2 Redox Titration of (6,5) Enriched Carbon Nanotubes

This example describes the basic observation of facile oxidation ofcarbon nanotubes in aqueous solution by a variety of strong oxidants,including KMnO₄, K₂IrCl₆, K₂IrBr₆, etc. The carbon nanotube materialsused here were those described in EXAMPLE 1. Aqueous solution of K₂IrCl₆was freshly made prior to use. To 100 μL of the nanotube solution inwater (OD_(990 nm)=0.96) was added the appropriate amount of K₂IrCl₆ sothat the final concentration of the oxidant was 0.25, 0.5, 1, 2 and 10μM, respectively. This resulted in a gradual decrease of the E11transition of the (6,5) tubes at 990 nm. The E11 intensity was found toreach a steady level a few minutes after the addition of the oxidant.FIG. 1A shows the UV-Vis spectra of the reaction mixture obtained 10 minafter addition of the various concentrations of K₂IrCl₆. Assuming thenanotubes were fully oxidized with 10 μM of K₂IrCl₆, and were in thefully reduced state without the oxidant, one could analyze the titrationdata using the Nernst equation. Such analysis yielded reductionpotential E_(CNT)˜800 mv (vs. SHE) for the (6,5) tube; and theconcentration of CNT [CNT]=1.9 μM.

Example 3 Redox Titration of Carbon Nanotubes with Large DiameterDistribution

This example demonstrates that the redox potential of carbon nanotubesvary systematically as a function of the tube diameter. The experimentwas similar to that described in EXAMPLE 2, except that non-fractionatedHiPco nanotube dispersed by DNA in aqueous solution was used. Bymonitoring the change of E11 transitions of different diameternanotubes, one could find that the larger diameter tubes (those atlonger wavelength) were oxidized first when lower concentrations of theoxidant K₂IrCl₆ were added; and smaller diameter tubes (those at shorterwavelength) were oxidized only after higher concentrations of theoxidant K₂IrCl₆ were added as shown in FIG. 1B. These results indicatethat the reduction potential of carbon nanotubes increases as the tubediameters decrease.

Example 4 Reduction of Oxidized (6,5) Carbon Nanotubes

This example demonstrates the reversibility of the carbon nanotubeoxidation. Oxidized carbon nanotubes were prepared by the addition of 10μM (final concentration) KMnO₄ in 100 μL of the (6,5) solution (OD₉₉₀nm=0.96). To this reaction mixture was added 1 μL of 1M NaOH. Thistriggered reduction of the oxidized nanotube by water, as illustrated bythe reaction:4 CNT⁺(oxidized)+H₂O→4 CNT⁻(reduced)+O₂+4H⁺

Within 10 min or so, the E11 transition of the (6,5) carbon nanotubes at990 nm was found to be fully recovered, indicating that the carbonnanotubes were fully reduced.

Example 5 Carbon Nanotubes as Catalysts for Redox Reactions

This example describes the use of carbon nanotubes as catalysts forredox reactions, as illustrated in the reaction shown below, where A andD refers to electron acceptor and donor, respectively:

The experiment used KMnO₄ as an electron acceptor, and water at pH 11.0as electron donor. To 100 μL of the (6,5) solution (OD_(990 nm)=0.96)was added 100 μM (final concentration) KMnO₄, then to this reactionmixture was added 1 μL of 1M NaOH. As a control experiment, 1 μL of 1MNaOH was added to 100 μL of 100 μM (final concentration) KMnO₄ solution.The consumption of KMnO₄ was monitored at 526 nm where KMnO₄ has anabsorption peak. As shown in FIG. 2, the reduction of KMnO₄ was greatly(˜100-fold) accelerated by the carbon nanotubes. FIG. 3 illustratesspectral changes of the KMnO₄ and CNT reaction mixture as a function oftime. A total of 24 spectra were taken over 6 hr period (15min/spectrum). FIG. 3 shows that carbon nanotubes were fully recoveredto the reduced state within 6 hrs or so.

Example 6 Dependence of Reduction Potential of the Carbon Nanotubes onDispersant Charge

HiPco carbon nanotubes (˜1 mg/ml) (Carbon Nanotechnologies, Inc.,Houston, Tex.) were dispersed by sonication for 60 min at 5 W in 30 mlof aqueous solution of 1 mg/ml Torula Yeast Type VI RNA (Sigma) in 0.1MNaCl plus 10 mM EDTA, pH 7. The dispersion was spun at 10,000 g for 30min. The supernatant was then pelleted by centrifugation at 540,000 gfor 1 h and washed by dispersion and recentrifugation in 0.1M NaCl plus10 mM EDTA, pH 7. HiPco carbon nanotubes (0.25 mg/ml) were alsodispersed by homogenization and sonication (cup horn sonicator for 1 hat 500 W) in 200 ml 1% Triton X-405 in water after which they werecentrifuged for 4 h at 141,000 g. The supernatant was concentrated usinga YM-100 filter in a Millipore-Amicon ultrafiltration cell. Absorptionspectra were recorded of the carbon nanotubes dispersions as is afterwhich spectra were recorded within minutes after the addition of 10 and100 μM K₂Ir(Cl)₆, a powerful oxidant with a reduction potential of 0.96V vs NHE.

FIGS. 4A and 4B show the absorbance spectrum of each set of carbonnanotubes without further treatment (upper RNA dispersed, lower TritonX-405 dispersed). After the addition of 10 μM K₂Ir(Cl)₆, the spectrum ofthe RNA-dispersed nanotubes was substantially changed with absorbancebands located between 900 and 1400 nm decreased in amplitude by close toone-half. Upon increasing the K₂Ir(Cl)₆ concentration to 100 μM thesesame absorbance peaks disappeared almost entirely. In the case of theTriton X-405 dispersed nanotubes, in the same wavelength range thechange in the spectrum was quite small upon addition of 10 μM K₂Ir(Cl)₆.Upon increasing the concentration of oxidant to 100 μM, the absorbancebands decreased in amplitude to no less than 50% of the initial value.

The much greater sensitivity of the RNA wrapped nanotubes to theaddition of oxidant indicates that these nanotubes are more easilyoxidized, having a lower reduction potential than those dispersed inTriton® X-405. The lower reduction potential of the RNA-wrappednanotubes is attributed to the presence of the anionic wrapping polymerwhich stabilizes the oxidized nanotube over what occurs with Triton®X-405, a non-ionic and therefore uncharged detergent.

Example 7 Reversibility of Redox Potential of Carbon Nanotubes

HiPco carbon nanotubes were suspended using Type VI RNA from Torulayeast (Sigma). One mg/mL carbon nanotubes (CNI Inc.) were sonicated inthe presence of 1 mg/mL RNA in 100 mM NaCl, 10 mM EDTA pH 7.0. Thedispersion was spun at 12,000 g for 10 min and stored at 4° C. FIG. 5shows a spectrum of the RNA-dispersed carbon nanotubes. The spectrumindicates that the longer wavelength components in the E11semiconducting region are about half oxidized just on standing in thepresence of air. The addition of the reductant, 10 mM sodium dithionite(Na₂S₂O₄) shows the restoration of the longest wavelength components totheir full amplitude. That the carbon nanotubes have become partiallyoxidized without the addition of exogenous oxidants means that they haveundergone partial oxidation due to oxygen in the air. The potential ofthe O₂/2H₂O redox couple is 0.82 V vs. NHE at pH 7.0. We know that at pH7.0 the nucleic acid wrapped-CNTs show a midpoint potential of around800 mV. The equivalent level of oxidation due to oxygen in the air at pH7.0 implies that the nanotubes are sensing spectroscopically thepresence of O₂ and its thermodynamically reversible redox potential.This observation, the oxidation by exogenous oxidants (e.g. KMnO₄ andK₂IrCl₆) and the reduction by dithionite indicates the presence of areversible electron transfer between carbon nanotubes and external redoxspecies.

Example 8 Carbon Nanotube pH Sensor

Because the O₂/2H₂O couple shows a pH dependence of 1H⁺/e⁻ whereas thecarbon nanotube does not show or shows less of a pH-dependent redoxcouple, the sensitivity of the CNT redox state to the O₂/H₂O coupleallows it to be used as a pH sensor. FIG. 6A shows the dependence of theabsorbance at 1290 nm on the pH using carbon nanotubes dispersed withthe non-ionic detergent Triton X-405. The same titration repeated in thepresence of increasing concentrations of SDS in the presence of TritonX-405, shown in FIG. 6B, shifted the titration to higher pH, consistentwith the shift of the reduction potential of the nanotube to lowervalues upon coating the nanotube with anionic charges. It is alsopossible that the increasing negative charge has an effect on the pHclose to the nanotube surface decreasing with increasing negative chargeof the dispersant. In both cases, the nature of the dispersant allowsone to tune the nanotube such that its absorbance spectrum tracks the pHover different ranges.

Example 9 Carbon Nanotube Photovoltaic Cell

This example demonstrates the construction of a carbon nanotube basedphotovoltaic cell. Such a device would operate though the followingphysical processes: 1. electrons in a carbon nanotube are excited bylight from the valence band to the conduction band; 2. excited electronsare withdrawn by an anode, which creates the oxidized state of thecarbon nanotube, which is then reduced by a redox mediator; 3. theoxidized mediator is in turn reduced back by the electrons provided by acathode. It would be expected that the wavelength range would be that ofnatural sunlight.

There are many possible embodiments of such a device. One type of such adevice is shown in FIG. 7. The conducting substrate 10, an opticallytransparent material, can be made with TiO₂; the carbon nanotube film iscomposed of a blend of semiconducting CNT and a suitable polymer isdeposited on the substrate to form the carbon nanotube substrate 20. Theconducting substrate 10 and nanotube substrate 20 together form theanode 60. The redox mediator 50 can be the 3I−/I₃− (iodide/triiodide)redox couple. The electrolyte 40 depends on the particular redoxmediator used, but can generally be any electrically conductive solutionthat is not easily reduced or oxidized, such as aqueous NaCl. The anodeand cathode 30 are interchangeable, depending on particular mediatorused.

Another embodiment of the device is a solid-state version of the oneshown in FIG. 7. Here, the electrolyte is replaced by a wide band gapinorganic semiconductor of p-type, such as CuI or CuSCN, or ahole-transmitting solid such as an amorphous organic arylamine.

Yet another embodiment of the device involves modification of the carbonnanotube surface with a dye sensitizer, such as the black dyetri(cyanato)-2′,2′,2″-terpyridyl-4,4′,4″-tricarboxylate)ruthenium (II).

1. A process to catalyze a redox reaction comprising: contacting atleast one carbon nanotube in solution with: a) an oxidizing agent and anoxidizable compound or b) a reducing agent and a reducible compoundwherein the redox reaction is catalyzed.
 2. The process of claim 1wherein the oxidizing agent is selected from the group consisting of2,3-dichloro-5,6-dicyano-p-benzoquinone, benzoyl peroxide, chloranil,chlorine, dichlorodicyanobenzoquinone, dichlorodicyano-p-benzoquinone,diphenylbenzoquinone, divalent chromium, formate, glucose,hexachloroiridate, hydrogen peroxide, hydroperoxide, hyperchlorous,hypophosphite, iodine,naphthoquinone, N-bromosuccinimide,N-chlorosuccinimide, N-iodosuccinimide, nitrate, nitric acid, nitrousacid, o-benzoquinone, o-chloranil, o-iodobenzoate, oxygen, ozone,p-benzoquinone, p-chloranil, peracetic acid, perborate, perboric acid,perchlorate, perchloric acid, permanganate, permanganic acid, peroxide,peroxyacids, perphosphoric acid, persulfate, persulfuric acid,pertitanate, pervanadate, p-toluquinone, ribose, saccharose, perforate,stannous, stilbenequinone, sulfite, sulfur, sulfuric acid,tetrathionate, trivalent titanium, and trivalent vanadium.
 3. Theprocess of claim 1 wherein the reducing agent is selected from the groupconsisting of 9,10-diphenylanthracene radical anion, acid chlorides,acridine anion, alkali metal sulfates, aluminum sulfates, anthraquinol,benzophenone anion, bisulfite, boranes, borates, carbon monoxide,chromium(II) salts, diimine, dithionite, formic acid, Group I & IImetals, hydrazine, hydrides, hydrogen, hydrosulfite, iron(II) salts,lithium aluminum hydride, low-valence titanium ions, naphthaleneradical-anion, nitrous acid, N-n-butylphthalimide anion, niobium(III)salts, perylene radical anion, phenanthridine radical-anion, phosphates,phosphines, phthalonitrile anion, polyphosphates, reducing sugars,sodium borohydride, sulfides, thiosulfate, thiourea, transition metals,trans-stilbene anion, and vanadium(II) salts.
 4. A process according toclaim 1 wherein the carbon nanotube is selected from the groupconsisting of single walled nanotubes and multiwalled nanotubes.
 5. Aprocess according to claim 1 wherein the carbon nanotube is in the formof nanotube ropes.
 6. A process according to claim 1 wherein the carbonnanotube is in the form of dispersed nanotubes.
 7. A process accordingto claim 1 wherein the carbon nanotube is associated with a dispersantor a mixture of disperants.
 8. A process according to claim 7 whereinthe dispersant is negatively charged.
 9. A process according to claim 7wherein the dispersant is positively charged.
 10. A process according toclaim 7 wherein the dispersant is uncharged.
 11. A process according toclaim 7 wherein the dispersant is a nucleic acid.
 12. A processaccording to claim 7 wherein the dispersant is a detergent.